In particular,
- Drop INCLUDES and LIBINC variables; referenced deprecated X11R6 directories
and INCLUDES unnecessarily included the default path.
- Switch hard-coded gcc for CC variable.
- Move hard-coded -02 and -Wall flags to new CFLAGS variable.
- Switch ${} to $() for consistency.
- Add new INSTALL_DIR variable for creating installation directories.
- Respect CPPFLAGS and LDFLAGS.